Those skilled in the arts of antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics characteristic of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different beams may be generated simultaneously.
Antenna arrays are becoming increasingly important for communication and sensing. Those skilled in the design of antenna arrays know that the physical size of the elemental antennas of the array and their physical spacing in an array is an inverse function of frequency, with higher frequencies requiring smaller antenna elements and spacings than lower frequencies. As it so happens, increasing bandwidths required for more sophisticated communications and sensing tend to result in the use of higher frequencies, with the result that the fabrication of antenna arrays tends toward fabrication of small structures arrayed with small inter-element spacings.
The problems associated with the fabrication of antenna arrays is exacerbated by the need which often occurs for the ability to radiate dual polarizations, which is to say the ability to selectively radiate or receive mutually orthogonal polarizations of electromagnetic energy, often termed Electric (E) and Magnetic (M) or Vertical “V” and Horizontal “H” polarizations, regardless of the actual orientations of the fields of the polarizations. The ability to receive (and to transmit) significantly in a given polarization depends upon having a “radiating aperture” in the direction of the electric field of the desired polarization. Thus, an antenna, in order to be an effective, should have finite (non-zero) dimensions (in terms of wavelength) in the direction of the electric field to be transduced. When dual polarization (or corresponding elliptical or circular polarization) is desired, the radiating elements must extend significantly in two mutually orthogonal directions.
The prior art relating to horn antenna arrays and their fabrication includes U.S. Pat. No. 6,891,511, issued May 10, 2005 in the name of Angelucci. The Angelucci method for fabricating an antenna array includes the placing an array of clips into a ground plane. The method also includes the “printing” of an array of electrically conductive horn antenna elements onto a first dielectric circuit board (or set thereof), which first board(s) define a slot adjacent each antenna element. Such a printed board has a significant dimension only in one plane, so can only be an efficient radiator in the plane of the board. The first board(s) are mounted in a mutually parallel manner on the array of clips. A second dielectric board (or set of boards) is printed with similar conductive horns, but its slots are arranged to mate with the slots of the first board(s). The second boards are mounted onto the clips and the first board(s) so that, when mated, the second boards are mutually orthogonal to the first boards, and the horns form a rectangular array in which the antenna elements of the first boards radiate in a first polarization, and the antenna elements of the second boards radiate in a second polarization, orthogonal to the first polarization. The physical arrangement of the clips tends to stabilize the antenna array against deformation attributable to dimensional stability deviations of the dielectric materials.
The prior art also includes U.S. Pat. No. 6,967,624, issued Nov. 22, 2005 in the name of Hsu et al., which discloses a wideband antenna element and an array made from such antenna elements. The antenna elements are defined on surfaces of dielectric plates, and the feed structure is defined on a second side of one of the plates. The plates are juxtaposed with the antenna portions in registry and the feed structure sandwiched between the plates. A strip conductor portion of the feed structure extends between the plates to allow the antenna element to be fed by an unbalanced conductor.
FIG. 1a is a simplified perspective or isometric view of a single horn antenna element 10 according to application Ser. No. 11/245,831. Antenna 10 defines a feed end 10FE, a radiating end 10RE, and an overall length L. In FIG. 1a, the antenna element 10 is comprised of two juxtaposed “printed-circuit” or dielectric boards, namely an upper board 12 and a lower board 14, each having width W. Each of the upper board 12 and lower board 14 defines a feed end 12FE and 14FE, respectively, and a radiating end 12RE and 14RE, respectively. FIG. 1b illustrates a feed-end view of the arrangement of FIG. 1a. Upper board 12 includes two portions, namely a dielectric board portion 12d and a metallic portion 12m. The upper surface of dielectric board 12d is designated as 12dus, and the lower surface is designated 12dls. In FIG. 1b, upper board 12 has left and right lateral edges 12le1 and 12le2. As illustrated in FIGS. 1a and 1b, the metallic portion 12m of printed-circuit board 12 overlies the upper surface 12dus of the dielectric portion of upper board 12. The metallic portion 12m is cut out to define a metal-free “through aperture” designated generally as 20 and an associated horn-defining slot 30 with “matching cavity” 31, as described in copending patent application 10/830,797, filed Apr. 23, 2004 in the name of Hsu et al. As illustrated in FIGS. 1a and 1b, upper printed-circuit board 12 partially overlies lower printed-circuit board 14. More particularly, the lower surface 12dls of board 12 overlies and is generally juxtaposed with upper surface 14dus of lower board 14. As also illustrated in FIGS. 1a and 1b, an aperture or slot 10a is defined in the near end of juxtaposed boards 12 and 14.
The description herein includes relative placement or orientation words such as “top,” “bottom,” “up,” “down,” “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” as well as derivative terms such as “horizontally,” “downwardly,” and the like. These and other terms should be understood as to refer to the orientation or position then being described, or illustrated in the drawing(s), and not to the orientation or position of the actual element(s) being described or illustrated. These terms are used for convenience in description and understanding, and do not require that the apparatus be constructed or operated in the described position or orientation.
As illustrated in the end view of FIG. 1b, the overlap or juxtaposition of boards 12 and 14 is only partial, in that the overlap extends only over a width of W−2t. That is, the overlap portion is not the full width W of the boards, but is instead less by twice the thickness t of the boards. At the left in FIG. 1b the left lateral edge 14le1 of bottom board 14 extends beyond the left lateral edge 12le1 of upper board 12 by thickness t, and at the right lateral edge 12le2 of upper board 12 extends past the right lateral edge 14le2 of lower board 14, also by thickness t. The presence of the overlap results in a “step” or “offset” 15 adjacent each long edge of the structure 10.
FIG. 2a is an exploded view of the arrangement of FIGS. 1a and 1b, illustrating boards 12 and 14 exploded away from each other to illustrate some details of board 14. In FIG. 2a, board 14 can be seen to be similar in size to board 12. The near or upper side 14dus of board 14 bears a pattern of metallization, corresponding to the feed arrangement for the horn of the arrangement of the Hsu et al. patent. More particularly, the pattern of metallization includes a strip conductor 16 which is a portion of a feed transmission line terminating at an end location 16e adjacent the juxtaposed feed ends 12FE and 14FE of the boards 12 and 14. The pattern of metallization also includes a capacitive or load portion 18, also described by Hsu et al.
FIG. 2b is a perspective or isometric view of the lower or reverse side of printed-circuit board 12 of FIGS. 1 and 2a, illustrating the dielectric lower surface 12dls, and a slot 12a cut part-way through the thickness t of the board 12d. The location of slot 12a is selected so that it overlaps or is registered with strip conductor 16 near its end portion 16e when boards 12 and 14 are juxtaposed as illustrated in FIGS. 1a and 1b. The purpose of the resulting slot or aperture 10a is to provide access for a feed pin or center conductor (not illustrated in FIGS. 1a, 1b, 2a, 2b, or 2c) when the horn antenna element 10 is formed by the juxtaposition of boards 12 and 14. The feed pin will then be immediately adjacent the end portion 16e of feed conductor 16.
FIGS. 3a and 3b illustrate the upper and lower sides, respectively, of a ground plane 300 suited for use with the horn antenna elements 10 as described in conjunction with FIGS. 1a, 1b, 2a, and 2b. FIG. 3c is a cross-sectional view of the structure 300 of FIG. 3a looking in the direction of section lines 3c-3c. FIG. 3d is a plan (overhead) view of the upper side of the structure 300 of FIG. 3a. The structure 300 should be electrically conductive, so it may be made from metal, as suggested by the hatching of FIG. 3c. However, in one embodiment, the ground plane 300 is made from metallized plastic. The upper surface 300us of ground plane 300 defines a plurality of elongated slots, extending (having their directions of elongation) in a first direction along the surface, some of which slots are designated 300S1. The upper surface also defines a further plurality of elongated slots 300S2 with their directions of elongation orthogonal to those of slots 300S1. The pattern of crossed slots 300S1 and 300S2 creates a plurality of rectangular or square “lands,” some of which are designated 300L in FIG. 3a. 
The bottom view of ground plane 300 in FIG. 3b shows a pattern of through apertures 300a extending from lower surface 300s. The apertures 300a extend through at least to the lower or bottom surfaces of the slots 300S1 and 300S2, and for ease of manufacture can extend completely through to the upper surface 300us. As illustrated in FIG. 3c, the lower surfaces of slots 300S1 are designated 300S1b. The apertures 300a form a rectangular pattern. The rectangular pattern of apertures 300a is registered with the sides of the lands 300L defined by the slots 300S1 and 300S2 on the upper side 300us of ground plane 300.
FIG. 3d is a plan view of the upper surface 300us of the ground plane 300 of FIGS. 3a, 3b, and 3c, showing how the mutually orthogonal slot sets 300S1 and 300S2 define a rectangular grid pattern defining lands 300L, and how the apertures 300a are centered on the sides of the lands 300L. As illustrated, the lands 300L are generally rectangular.
The through apertures 300a are provided to act as connector shrouds for accepting coaxial feed connectors applied from the lower side of the ground plane 300. For this purpose, each aperture 300a is fitted with a pin having its axis oriented parallel with the axis of the aperture. In order to carry electromagnetic signals in a guided coaxial mode, the pin must be supported by dielectric. FIG. 4 is similar to FIG. 3c, with the addition of pins 410 extending axially through the apertures 300a, supported in position by dielectric pieces 412. The dielectric pieces 412 can be glass fused to both the interior surfaces of the apertures 300a and to the exteriors of the pins 410, or they can be any other convenient dielectric support. Naturally, the dimensions of the pins 412 and the interior diameters of the apertures 300a at locations near the lower surface 300s of ground plane 300 must be selected to mate with a corresponding connector, preferably an inexpensive standard connector type such as SMA. The diameter of the pins 410 near the upper side 300us of the ground plane 300 should be selected to provide a tight or interference fit into the aperture 10a in the feed end 10fe of the antenna 10 of FIG. 1. Ideally, the same diameter is selected to meet both these requirements. The projection of the pins 410 into the slots 300S1 or 300S2 of FIG. 4 is selected to extend into the aperture 10a, but not to bottom therein.
The two dielectric halves of each horn antenna are fastened together in the offset-juxtaposed manner illustrated in FIG. 1a, as by fusion bonding or welding, or by application of adhesive. If adhesive is used, it can be applied in liquid form and allowed to harden or cure. A suitable adhesive material may be epoxy resin. The fusion bonding or welding or the adhesive is performed or applied, as applicable, to those portions of the lower surface 12dls of board 12 and of the upper surface 14dus of board 14 which are juxtaposed as illustrated in FIGS. 1a and 1b. The conjoined board portions 12 and 14 together form a single horn antenna 10 capable of being fed at the feed end 10FE and radiating at the radiating end 10RE (remembering that the antenna is reciprocal in its operation).
In order to make an array antenna, a plurality of individual horn antennas such as 10 of FIGS. 1a and 1b are produced or procured. A baseplate or ground plane 300 similar to that of FIGS. 3a, 3b, 3c, and 3d is also procured, with pins inserted as illustrated in FIG. 4.
The principles by which the individual horn antennas such as 10 of FIGS. 1a and 1b are arrayed are illustrated with the aid of FIGS. 5a, 5b, 5c, and 5d. FIG. 5a is a top isometric view of an assembly 500 of four horns 10, FIG. 5b is a bottom isometric view of the assembly of FIG. 5a, and FIGS. 5c and 5d are bottom views of an assembly 500 of four horn antennas 10 of FIGS. 5a and 5b at different stages of fabrication of the array. In order to fabricate the horn antenna array, each individual horn antenna 10 is conceptually juxtaposed with three other like horn antennas 10, with their steps or offsets 15 linked to form an “X” shape in end view, as illustrated in FIG. 5c. The four juxtaposed horns are then inserted into a slot crossing of the ground plane, as for example at the crossing of slots such as 300S1 and 300S2 of FIG. 3a. Additional four-horn assemblages 500 are added to the ground plane 300, fitting their steps 15 into the steps 15 of already-added four-horn assemblages 500, to form a complete horn array structure 600, at least a portion of which has the general appearance illustrated in FIG. 6. While it is conceptually appealing to view the assembly of array 600 in this manner, a possibly more practical technique is to use pick-and-place machinery to pick up individual horn antennas 10, and to individually place them in open slot positions in the baseplate. Pick-and-place machinery is well known and widely used, and those skilled in the art know how to use the technique.
During the assembly of the individual horn antenna elements 10 into the structure 600 of FIG. 6, the pick-and-place, whether performed by hand or by machinery, must be such as to fit the appropriate one of the pins 410 of FIG. 4 into the aperture 10a in the feed end 10FE of the corresponding horn antenna 10. FIG. 7a illustrates the relationship which should be maintained between a feed pin 410 and the feed conductor portions 16e and 16 of a dielectric board 14, and FIG. 7b illustrates the relationship which should be maintained between the feed pin 410 and the aperture slot 10a of board 12. In general, the pin 410 must be juxtaposed with, and preferably centered on, conductor portion 16e. Also, the pin 410 should not “bottom” in slot 10a, lest its presence prevent the horn antenna 10 from being held in its correct position.
Once all the pick-and-place has been accomplished to form a structure 600 similar to that of FIG. 6, reflow soldering (or possibly other fusion jointing) is performed on the entire assemblage. For this purpose, portions of the metal which are to be fused or soldered are “tinned” before assembly. Those skilled in the art know that tinning refers to pre-coating with a material which facilitates the fusion bonding process. The pre-tinned assemblage 600 is placed in a hot environment until the fusion material melts and flows, with the result that surface tension effects cause the various portions of the fusion material to fuse together. A bottom view of four mutually adjacent horn antenna elements 10 is illustrated in FIG. 5d, with the result of the reflow soldering or fusion illustrated as an interlaced joint 550 with solder. The assemblage is then removed from the heat and allowed to cool, with the result that the structure 600 becomes monolithic or one piece.
It will be noted that the various horn antennas 10 which are initially assembled to the baseplate or ground plane, before the soldering or fusion to make a monolithic structure, are held only at their bottoms by virtue of insertion of their feed ends into the slots of the baseplate. This may allow some play at the radiating ends of the horns as assembled into the array, which in turn may tend produce imperfect results. A jig or fixture is assembled onto the radiating ends of the horn antennas assembled into the array, to thereby fix the radiating ends of the horn antennas as well as the feed ends.
FIG. 8a is an isometric view of an array 600 of horn antennas 10 assembled onto a baseplate or ground plane 300, much as shown in FIG. 6, with the addition of a solder fixture 810 for holding the radiating ends of the horns of the array. For holding the radiating ends of the horns 10 of the array 600, solder fixture 810 is provided with mutually orthogonal or crossed slots, substantially equivalent to the antenna-receiving slots in the upper side of ground plane 300. These slots in the solder fixture mate with the boards of the various antennas 10 of the array, and hold them in fixed position at the top. Thus, the horn antennas 10 of the array 600 of FIG. 8a are held in proper position at both their tops and at their bottoms before soldering. In order to be most effective, it is desirable that the fixture 810 be readily removable after the soldering operation is finished, for which purpose the fixture 810 is made from a material, such as graphite, which resists wetting by the solder.
The antenna holding fixture 810 of FIG. 8a is fitted with reservoirs or means for holding solder balls. These solder balls provide a reservoir of molten solder during the reflow soldering operation to fill in any areas which might otherwise have solder gaps. In the arrangement of FIG. 8a, the reservoirs are illustrated as a set of apertures 812. These apertures are located over the “X” joint of each set of four juxtaposed horn antennas, most easily seen in FIGS. 5c and 5d. The reservoir apertures 812 communicate by way of funnel sections 814 with the upper portion of the juxtaposed horn antennas 10 of each set of four horn antennas, as illustrated in FIG. 8b. The heating associated with the reflow soldering is performed with the solder fixture 810 in place and with a ball of solder 814 in each reservoir 812. When the reflow temperature is reached, not only does the “tinning” solder melt, but so do the solder balls 814. Gravity and surface tension help the solder flow from the melted balls in the reservoirs 812 to help in filling the region between the juxtaposed steps 15 of the horn antennas 10 of the array 600.
After assembly of the horn antenna array 600 and making it monolithic, standard coaxial fittings, such as SMA fittings, or any other type, can be affixed to the apertures 300a and pins 410 from the bottom side 300ls of the ground plane 300.
Improved or alternative antenna arrays and methods for fabrication thereof are desired.